The present invention relates to an injection molding machine for manufacturing plastic moldings and including a plurality of hydraulic loads for individual subassemblies, and also to a method of controlling hydraulic drives of the foregoing type of injection molding machine.
From EP 0,201,610 A1 an injection molding machine is known in which a servomotor drives a toggle-lever closing unit. In that case, the motor acts via a chain on the nut of a ball spindle.
With that construction, a linear movement is produced mechanically by rotation, with the disadvantage of unavoidable wear of the mechanical parts. In addition, the speed of rotation of the drive unit is limited in order to avoid damage to the ball-spindle system and in order to prevent loud operating noises.
From Federal Republic of Germany OS 3,731,021 an injection molding machine with hydraulic loads of individual subassemblies is known. The hydraulic systems of such machines ordinarily have a central pressure-producing station in which an electric motor drives a fixed-displacement pump and the pressure and volume control of the hydraulic system is effected by valves.
In addition to the annoyance by noise due to the continuous operation of motor and pump, such drives of injection molding machines have the disadvantage of consuming a large amount of energy. In addition, the control behavior of these hydraulically operated injection molding machines is also relatively sluggish, even upon the use of servovalves and control pumps.
It is an object of the present invention to provide hydraulic drives and a method of controlling said drives in injection molding machines which, while being of compact construction, move the individual subassemblies with strong power, rapidly and in noise-reducing fashion, as well as with less consumption of energy.
The foregoing object is achieved by an injection molding machine for manufacturing plastic moldings in accordance with the invention. Such machine comprises a plurality of hydraulic loads for individual subassemblies including a closing unit, an injection unit, an ejector and cores. A plurality of pumps are respectively connected to said hydraulic loads; and at least one alternating-current servomotor is provided for driving the pumps.
In a method for controlling hydraulic drives of the foregoing injection molding machine, a measured value of a subassembly is detected. Such measured value is compared with the desired value, and any deviation detected thereby is converted into a desired velocity value. The speed of rotation of an associated motor is then adjusted, via a servodrive module, in response to the desired velocity value.
The individual loads are connected directly to a pump which is driven by a servomotor. A uniform drive concept is produced since the drives of all subassemblies are driven hydraulically regardless of whether they perform a main function or a secondary function.
The energy required for driving the hydraulic loads is produced only when required since the servomotor can operate in a so-called stop-and-go mode. The work energy produced by the pumps is fed in simple fashion to the loads by pipe lines, without intermediate switching of hydraulic control elements such as directional or relay valves, and converted into linear motion.
Installation of these drives in existing drive systems is possible without any particular expense.
The pre-tensioning pressure required in the closed hydraulic system can be maintained in a structurally simple manner by pressure-maintaining pumps or by standardized accumulators. The transmission of force from the producer to the load is effected with extremely high speed and is not limited by mechanical parts. The result is a cycle time which is much shorter than that of electrical-mechanical injection molding machines.
The hydraulic system is almost free of wear. The mechanical parts used have self lubrication as a result of the fluid used, i.e. oil. In addition to this, hydraulic systems are not as strong as a mechanical system, so that extreme load peaks are avoided. Nevertheless, these hydraulic systems are of high rigidity which permit reproduceable values with respect to the path or force. This advantage is further increased by the direct transfer of power within the closed hydraulic circuit and therefore without the disturbing intermediate switching of valves and the like.
The number of structural parts necessary for the loads of the injection molding machine can be reduced in advantageous fashion or, for the same number, can be replaced by simpler parts. Thus a servomotor including a subcontrol circuit can be replaced by simple switchable clutches which are arranged between motor and pump. The advantage of the pump size which is adapted to the subassembly is retained thereby.
Instead of the switchable couplings, more than one pump can, in another advantageous arrangement, be connected to a servomotor.
By the use of variable displacement pumps, the volumetric flows of the loads of the individual subassemblies can be freely adjusted, both in volume and in direction. The control system of these standardized variable displacement pumps is of simple construction and is adapted to the control system of the servomotors.
The high dynamics which the hydraulics have as a result of the system is further supported by the procedure upon the regulation of the injection molding machine of the invention. Within a sub-system, namely a position or force control circuit, the measured values ascertained on the corresponding subassembly are controlled and converted into a velocity value. Within the control circuit of the drive motor, this velocity value is used as a desired value and is compared directly with the actual speed of rotation of the pump shaft and adjusted by a servovalve module in case of deviation.
This method of control permits a rapid analog response to changes in the actual behavior of the subassemblies.